Revised handling of TIFFTAG_INKNAMES and related TIFFTAG_NUMBEROFINKS value

In order to solve the buffer overflow issues related to TIFFTAG_INKNAMES and related TIFFTAG_NUMBEROFINKS value, a revised handling of those tags within LibTiff is proposed:

Behaviour for writing:
    `NumberOfInks`  MUST fit to the number of inks in the `InkNames` string.
    `NumberOfInks` is automatically set when `InkNames` is set.
    If `NumberOfInks` is different to the number of inks within `InkNames` string, that will be corrected and a warning is issued.
    If `NumberOfInks` is not equal to samplesperpixel only a warning will be issued.

Behaviour for reading:
    When reading `InkNames` from a TIFF file, the `NumberOfInks` will be set automatically to the number of inks in `InkNames` string.
    If `NumberOfInks` is different to the number of inks within `InkNames` string, that will be corrected and a warning is issued.
    If  `NumberOfInks` is not equal to samplesperpixel only a warning will be issued.

This allows the safe use of the NumberOfInks value to read out the InkNames without buffer overflow

This MR will close the following issues:  #149, #150, #152, #168 (to be checked), #250, #269, #398 and #456.

It also fixes the old bug at http://bugzilla.maptools.org/show_bug.cgi?id=2599, for which the limitation of `NumberOfInks = SPP` was introduced, which is in my opinion not necessary and does not solve the general issue.